Thermochemical gasification of lignocellulosic materials (biomass feedstocks) has conventionally been limited to relatively "dry" feedstocks (i.e., under 50% moisture by weight). One reason for this constraint is the high latent heat of vaporization of water, which places limitations on gasification temperatures in high-temperature, short-residence time gasifiers. Direct combustion routes to recovering stored solar energy from biomass are similarly constrained, since the moisture content directly affects the net heating value of the biomass fuel.
Because of this practical limitation on moisture content, the range of feedstocks used for thermochemical processing has been narrow. Wood has been the primary feedstock, although various crop residues and by-products have been tried. While field drying is feasible in some instances, often these materials must be force-dried to some extent, which has the effect of reducing the net energy produced by the system.
Many otherwise promising biomass feedstocks have prohibitively high moisture contents. These include aquatic plants having moisture contents in excess of 90%, and many crop residues and tropical grasses having moisture contents as high as 75-85%. Some of these feedstocks are quite attractive as potential energy crops because they exhibit high growth rates, are relatively easy to harvest, or perform some beneficial ancillary function. For example, water hyacinths can be used to reduce the organic and heavy metal content of sewage water streams.
The high growth rate and availability of some of these high-moisture feedstocks make them potentially valuable additions to the resource base, provided that an efficient system for recovering their stored energy can be developed. The Department of Energy, recognizing the potential of these feedstocks, supports a program devoted to the development and study of herbaceous crops for production of biomass energy. The Department of Agriculture is also studying a number of plants which can be grown on marginal soils to produce chemicals and energy. Many of these materials are simply too wet to be used as a feedstock in conventional high-temperature thermochemical gasification systems. In addition, the ash contents of some high moisture feedstocks is quite significant and could pose problems in a high-temperature gasification system.
In addition to cultivated feedstock sources, a wide variety of food processing waste streams contain lignocellulosic materials which could be converted into fuel gas. Solid and liquid wastes, such as brewer's spent grain, tomato cannery wastes, potato peeling and processing wastes and grape pomace, are potential feedstocks if the high moisture content can be overcome.
Some of the feedstocks mentioned above have also been studied for possible biological conversion of the biomass to methane, such as by anaerobic digestion, which is an inherently high-moisture operation. Biogasification has some serious limitations, however. Compared to thermochemical gasification, the rate of conversion is orders of magnitude slower. The most direct process implication of this lower rate is the relatively large reactor volumes required to produce a given quantity of gas. The ultimate degree of conversion which can be obtained in biological systems is also limited, particularly in materials with high lignin contents. Finally, biological gasification systems are more susceptible to process upsets due to control system failure or the inadvertent introduction of poisons to the reactor.
Modell et al. U.S. Pat. No. 4,113,446 shows a gasification process for converting liquid or solid organic material to gas. The patent, while claiming that catalysts are not required in the conversion, lists suitable catalysts as including nickel, molybdenum, cobalt, their oxides and sulfides, and noble metal catalysts such as platinum, palladium or the like or mixtures thereof either unsupported or supported on a base such as silica, alumina mixtures thereof or the like. Although Modell et al. mention that any organic solid material, including garbage, paper, sawdust, etc. can be converted using process parameters similar to those in the present application, the test results shown in the patent and elsewhere in the open literature show that a suitable catalyst was never identified for these feedstocks. There is no indication that Modell et al. ever experimented with combining catalysts to increase the conversion rate. In addition, there is no indication that Modell et al. appreciated the effects of alkali as a catalyst for the reaction or understood the importance of reduced nickel metal as opposed to the metal oxides and noble metals claimed as catalysts in the patent. Since Modell et al observed no catalytic activity in their system, this fact led them to believe that the conversation could be performed in the presence or absence of a catalyst.